Collier finds hollow alluvial rock (7)
I believe the answer is:
mineral
'rock' is the definition.
'collier finds hollow alluvial' is the wordplay.
'collier' becomes 'miner' (I've seen this before).
'finds' is a charade indicator (letters next to each other).
'hollow' says to hollow out the word (remove centre letters) (the word is hollowed out).
'alluvial' with its centre removed is 'al'.
'miner'+'al'='MINERAL'
(Other definitions for mineral that I've seen before include "Substance needed for health; ore" , "Ore; soft drink" , "What drink is available in ground" , "Gypsum or magnesium, say" , "Gypsum or magnesia, say" .)